1. Field of the Invention
This invention relates to a liquid photosensitive resin composition and, more particularly, to a pattern-forming liquid photosensitive resin composition which can be used as a solder resist (or solder mask) in the fabrication of printed circuit boards.
2. Description of the Prior Art
Conventionally, a solder resist (or solder mask) is widely used in the fabrication of printed circuit boards in order to form a permanent protective coating for printed circuit boards. Such a solder resist is used for the purpose of preventing the formation of a solder bridge during soldering and of ensuring the protection of the conductive parts against corrosion and the retention of their electrical insulation during use. As is evident from their purpose of use, solder resists for use under severe conditions, unlike etching resists, must have the following characteristics:
(a) Retention of adhesion during soaking in solder. (at 240.degree. to 280.degree. C.) PA0 (b) Permanent retention of adhesion. PA0 (c) Excellent resistance to solvents and chemicals. PA0 (d) Retention of good electrical insulating properties under high-humidity conditions. PA0 (a) 10 to 55% by weight of an at least partially carboxyl-modified multifunctional epoxy acrylate and/or multifunctional epoxy methacrylate having an average acid value of 4 to 150 and a number average molecular weight of not greater than 5,000, PA0 (b) 10 to 55% by weight of an acrylic and/or methacrylic cross-linking monomer and/or oligomer other than the epoxy acrylate and/or epoxy methacrylate, PA0 (c) 5 to 55% by weight of an acrylic and/or methacrylic monoethylenically vinyl monomer, PA0 (d) 4 to 35% by weight of an inorganic filler, and PA0 (e) 0.05 to 20% by weight of a photo-initiator and/or photosensitizer.
In order to meet these requirements, it has been conventional and common practice to form solder resists by screen printing of a thermosetting ink or a photocurable ink. In recent years, however, the increasing miniaturization of printed circuits has created a demand for solder resists having large coating thickness and high precision, and it is the existing state of the art that the screen printing method for the formation of solder resists is no longer satisfactory from the viewpoint of precision and coating thickness.
The formation of solder resists by the development technique has been proposed as a means for responding to this miniaturization of printed circuits. According to the development technique, a pattern is formed by applying a liquid photosensitive resin composition, or laminating a photosensitive film, to a printed circuit board, exposing the coating layer to active radiation through, for example, a photomask to cure only desired portions thereof, and then washing the coating film with a developer to remove any uncured portions thereof. This technique makes it possible to form a solder resist pattern having large coating thickness and high precision.
According to the manner in which the coating film to be cured is formed, solder resists for use in the development technique can be classified into three types: dry film type, solvent evaporation type and solventless liquid type. Among them, solder resists of the dry film type, as disclosed in Japanese patent laid-open No. 1018/'79, have the disadvantage that, in order to cause the solder resist to adhere closely to an uneven surface having a circuit formed thereon, a special process such as heat lamination under reduced pressure is required as suggested in Japanese patent laid-open No. 52703/'77 and that, notwithstanding the use of such a process, perfect adhesion is not always ensured. In contrast, solder resists of the solvent evaporation type, as disclosed in Japanese patent laid-open No. 15733/'76, can exhibit good adhesion to an uneven surface having a circuit formed thereon. However, they have the disadvantage that, after the application of a liquid photosensitive resin composition, the resulting coating film must be dried in an explosion-proof dryer or similar equipment so as to evaporate the solvent. Accordingly, there is a great need to develop an improved photosensitive resin composition of the solventless liquid type for use as a solder resist.
On the other hand, liquid photosensitive resin compositions can also be classified according to the type of developer used. They include ones using an organic solvent such as 1,1,1-trichloroethane and ones using a dilute aqueous alkaline solution. Since the use of an organic solvent involves problems concerning the pollution of the working environment and the disposal of waste liquid, development with a dilute aqueous alkaline solution is highly desirable.